Fluid flowmeters of the pressure drop or differential type are well known. Orifice plate and venturi flowmeters are probably the most common of the pressure drop type. Orifice plate flowmeters are inexpensive, but they are inherently high energy loss devices since the measured pressure drop across the orifice is non-recoverable, i.e. the drop in pressure is a drop in total pressure. Venturi flowmeters are low energy loss devices relative to orifice plate flowmeters since most of the pressure drop in the venturi throat is recoverable at the venturi outlet, i.e., the drop in pressure in the throat is due to an increase in kinetic energy of the fluid. However, when either of these flowmeters are used to measure fluid flow which varies over a wide range, such as air flow to an automotive engine, they either overly restrict total airflow at high engine speeds and loads if they are sized small enough to provide an adequate differential signal at low engine speeds and loads, or they provide an inadequate differential pressure signal at low engine speeds and loads if they are sized larger.
One prior art patent proposed a fluid flowmeter having a main air passage and a relatively small venturi tube in the main air passage receiving a portion of the total air flow and providing a static pressure signal for determining volumetric air flow in conjunction with a stagnation pressure signal in the main passage. This same patent also proposed placing a restriction in the main passage between the venturi tube inlet and outlet to increase the pressure difference across the venturi and thereby increase the pressure difference between the static and stagnation pressure. However, the restriction has the disadvantage of increasing the total pressure drop across the flowmeter, thereby increasing energy loses and decreasing the operating range of the flowmeter.
An additional shortcoming of many prior art devices resides in the fact that they are dedicated to a particular application and cannot be easily adjusted or reconfigured to accommodate differing applications or operational variations in a given application from system to system. For example, many flowmeters intended for automotive application are designed for an engine of known displacement and idealized respiration characteristics. Such flowmeters are often unsuitable for engines of slightly differing displacement or engines of the same displacement which fall in the outer fringe of design tolerances.
One prior art approach to affectively increase the operating range of a flowmeter while maintaining an acceptable pressure signal level is the bypass, which operates to shunt some of the fluid flowing through the meter around the swirl vanes, orifice, venturi tube or other signal generating element therein. Although such devices extend the range of operation, they have two major shortcomings. First, an inherent error factor is invited when the totality of air flow is not measured inasmuch as the ratio of measured air flow to bypassed air flow may vary. Additionally, such devices add mechanical complexity with its incumbent cost, reduced response (due to the mass of the moving parts) and shortened lifetime (due to the exposure of the interface between moving and nonmoving parts to moisture and contaminents within the air flow).